Metal cans, one mode of packaging container for food and drink, are superior in mechanical strength and permit long term preservation of contents. In addition, they allow easy packing of contents at a high temperature and sealing of the contents, as well as a sterilization treatment by retorting and the like. Therefore, they offer high reliability as to the safety and hygienic conservation in a packaging container. In view of many advantages they offer, various contents are packed in cans and used in great numbers in recent years, because they can preserve contents in a heating state and make separating collection of cans after use relatively easy.
The inside and outside of metal cans for food and drink are generally coated with a paint containing a thermosetting resin as a main component, with the aim of preserving flavor of contents, preventing corrosion of metal cans, improving appearance and protecting printed surfaces of the outside of cans. Because such metal cans are produced using great amounts of solvents, environmental problems due to desolvation during production and hygienic problems due to the solvents remaining in the coated film are inevitable. In addition, the flavor may be degraded due to the oligomer remaining from a reaction failure during heat setting.
In an attempt to solve these problems, a method including laminating a plastic film on a metal has been proposed. A laminate of a plastic film on a metal sheet is proposed for the application to so-called a three-piece can (hereinafter to be abbreviated as 3P can) and a two-piece can (hereinafter to be abbreviated as 2P can). Spreading of 2P cans is desirable for the production of seamless cans.
2P cans are generally produced by punching out a metal laminate consisting of a metal sheet and a plastic film with a can forming machine and draw-wipe forming. In this can forming step, the film is required to follow spread of the metal sheet while standing the shear of draw-wipe forming.
To meet the request, a film superior in can forming performance, which is made from a polyethylene terephthalate (PET) polyester resin having a specific intrinsic viscosity and a polybutylene terephthalate (PBT) polyester resin having a specific intrinsic viscosity has been proposed. However, this dramatic film does not necessarily follow spreading of a metal sheet, nor does it show a sufficient adhesion to a metal sheet, when subjected to a wipe step (draw-wipe forming) after drawing. In addition, the film is subject to delamination or development of microcracks during forming, thus showing insufficient formability.
The amount of oligomer eluted from a laminated metal sheet and a metal can obtained using this film is considerably less as compared to that from a metal sheet or a metal can obtained by applying a paint containing a thermosetting resin as a main component. For a further decrease, a metal container can formed from a metal sheet laminated with a film may be heat treated to increase the degree of crystallization of the film, but rapid crystallization results in the production of spherulite, delamination of the film and the like, which require demanding an improvement. A merely high degree of crystallization makes thermocompression bonding of the metal sheet and the form-processing difficult.
A plastic film having suitable crystallinity (suitable degree of crystallization), which is made from two kinds of polyester has been proposed to solve the above-mentioned various problems. This film simultaneously satisfies followability to the spreading of a metal during can forming and retention of the flavor of the can contents, which is attributable to the fact that the film is made from two kinds of polyesters and the film has suitable crystallinity. The film first has suitable crystallinity, but a treatment of the film at a high temperature causes transesterification leading to copolymerization with different polyester, thus degrading the crystallinity. To keep the degree of crystallization of the film in a suitable range during processing into products, for example, the time for melting the polyester may be shortened and the heat applied to the film during drawing and a heat treatment step after melting may be reduced.
For the productivity at an industrial scale, these conventional methods are faced with a limitation. This is because the amount of a resin to be discharged needs to be increased for improving productivity, a higher capacity of an extruder is necessary for producing a plastic film free of radical variation in quality such as inconsistent thickness, and the residence time of the resin needs to be prolonged to stabilize the discharge, and the like. The above-mentioned conventional methods are insufficient to achieve the productivity to afford a plastic film having the necessary properties and a metal can and the like obtained therefrom.
Given the above situation, there is a demand on a plastic film for a metal laminate, which has the properties necessary for a metal can and a lid of a can, which maintains the property after form processing into a final product, and which is superior in productivity at an industrial scale.
The present invention aims at achieving the following objects.    (i) Provision of a film for a metal laminate, which is superior in mechanical property, capable of thermocompression bonding with a metal sheet, despite a high degree of crystallization of the film, reduces quality change of plastic film laminated on the metal sheet even if faced with variation of conditions of thermocompression bonding with the metal sheet, and which is capable of thermocompression bonding even at a relatively low temperature.    (ii) Provision of a film for a metal laminate, which is superior in formability (can forming performance and the like).    (iii) Provision of a film for a metal laminate, which is superior in flavor property and impact resistance, and which obliterates occurrence of whitening or delamination of the film, or microcracks on the film, even if a film on the surface of a metal laminated sheet obtained by thermocompression bonding of the film on the metal sheet or on the surface of a metal can obtained by subjecting the metal laminated sheet to various form processings is crystallized.    (iv) Provision of a film for a metal laminate, which satisfies productivity at an industrial scale.